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cmd/link: insert trampolines for too-far jumps on ARM

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ARM direct CALL/JMP instruction has 24 bit offset, which can only
encodes jumps within +/-32M. When the target is too far, the top
bits get truncated and the program jumps wild.

This CL detects too-far jumps and automatically insert trampolines,
currently only internal linking on ARM.

It is necessary to make the following changes to the linker:
- Resolve direct jump relocs when assigning addresses to functions.
  this allows trampoline insertion without moving all code that
  already laid down.
- Lay down packages in dependency order, so that when resolving a
  inter-package direct jump reloc, the target address is already
  known. Intra-package jumps are assumed never too far.
- a linker flag -debugtramp is added for debugging trampolines:
    "-debugtramp=1 -v" prints trampoline debug message
    "-debugtramp=2"    forces all inter-package jump to use
                       trampolines (currently ARM only)
    "-debugtramp=2 -v" does both
- Some data structures are changed for bookkeeping.

On ARM, pseudo DIV/DIVU/MOD/MODU instructions now clobber R8
(unfortunate). In the standard library there is no ARM assembly
code that uses these instructions, and the compiler no longer emits
them (CL 29390).

all.bash passes with -debugtramp=2, except a disassembly test (this
is unavoidable as we changed the instruction).

TBD: debug info of trampolines?

Fixes #17028.

Change-Id: Idcce347ea7e0af77c4079041a160b2f6e114b474
Reviewed-on: https://go-review.googlesource.com/29397
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Cherry Zhang 2016-09-14 14:47:12 -04:00
parent d03e8b226c
commit 7c431cb7f9
16 changed files with 359 additions and 112 deletions
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1
src/cmd/link/internal/ld/main.go
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@ -86,6 +86,7 @@ var (
FlagW = flag.Bool("w", false, "disable DWARF generation")
Flag8 bool // use 64-bit addresses in symbol table
flagInterpreter = flag.String("I", "", "use `linker` as ELF dynamic linker")
FlagDebugTramp = flag.Int("debugtramp", 0, "debug trampolines")
FlagRound = flag.Int("R", -1, "set address rounding `quantum`")
FlagTextAddr = flag.Int64("T", -1, "set text segment `address`")